Although not as deftly written as the Uglies series, it's definitely better than his 'Risen Empire' attempt at adult fiction.
Leviathan is shamelessly steampunk...with a twist. The Clanker powers use zeppelins and mecha (which they call walkers).
The Darwinist powers, which include Britain, use genetic engineering. To fight World War I. That's how far alternate this history is. Despite this, he manages to do a better job of realistic alternate history than recent Turtledove.
Leviathan also jumps on the popular YA romance bandwagon. Deryn Sharp is a young woman who only wants to fly - so she dresses up as a boy and enlists as a midshipman in Britain's air force. Aleksandr is...the heir to the Austro-Hungarian Empire. It all seems rather unlikely...and obviously I'm going to stop there as I'd be introducing spoilers.
The hardcover edition at least also has something wonderful...it's illustrated. It's so rare these days to find an illustrated volume that's not intended for young children. Not only that, but it's illustrated in the way a book published in 1914 would have been illustrated - the illustrations aren't actually woodcuts, but they do a good job of faking it, and they are just absolutely amazing. So, I'm going to give a shout out to Keith Thompson.
Oh, and watch out for the flechette bats and their rather, uh, unique way of attacking enemies.